Using the data from page 2, I ran a few numbers on the top 25 firms ranked by revenue. I knew that it was big, but I never realized the incredible magnitude difference between the Big 4 Accounting Firms and everyone else in the market.

Average Revenue:Big 4: $6,167MMNext 10: $473MMBottom 11: $109MM

Average number of partners:Big 4: 2,079Next 10: 253Bottom 11: 61

Average number of professionals:Big 4: 18,245Next 10: 1,809Bottom 11: 482

Average number of offices:Big 4: 96Next 10: 38Bottom 11: 10

Average professionals per office:Big 4: 189Next 10: 52Bottom 11: 62

Average SEC Clients:Big 4: 1,596Next 10: 111Bottom 11: 27

In fact, if you combined the 5th through the 25th ranked firm, you still wouldn't equal one of the average Big 4 firms. Instead, you just have a lot more partners/professionals/offices producing lower revenue.

As you can see, there are some incredible economies of scale for the Big 4 firms. It appears that much of it is related to the number of SEC clients. The Big 4 have a combined total of 6,383 vs only 1,407 for the next 21 biggest CPA firms.

If you add together the number of Partners with the number of Professionals (assuming that they are almost entirely CPAs) of the Big 4, you get 81,297. So, roughly 18.5% of all CPAs are employed by the Big 4. Another 26,594 are employed by the next 21 firms ranked by revenue or about 6%. The total for the Top 25 is 107,891 employees. The AICPA states that 133,379 of it's members are employed in public accounting. Using the same 3/4 ratio of members/actual, this means there are a total of 177,839 CPAs total in public accounting. Of this number, 61% work for the top 25 firms and 45% work for the Big 4 alone.